If you're in their signature campaign and participants in the signature is much, just calculate the stakes and do your calculations. If it doesn't favour you, leave and if you think it will, continue it.
Most of the signature campaign doesn't allow leaving in mid of campaign! If you leave your whole works/stakes will be abandoned! That's why people hate extension in bounty campaigns! Though some bounty manager realizes this situation, that's why nowadays they let participants leave the signature after 4 weeks or after the first phase of bounty campaign!
Little bit disagree with your statement. I don't think most of the signature campaign doesn't allow leaving in mid of campaign. But it's true that some campaigns don't allow it which is really uncomfortable situation for hunters. And yes specially those BM who are really reputed by their work they will give the chance to bounty hunters to leave in mid of signature campaign.